This invention relates to a method for use in controlling the operation of a steerable drilling system. The method is particularly suitable for use with a rotary steerable system, but may be used in other types of steerable drilling system used in the formation of subterranean wells.
One type of rotary steerable system comprises a downhole assembly including a drill bit. The drill bit is carried by a drill string which is rotated typically by a well head located drive arrangement. A bias unit is included in the downhole assembly, the bias unit including a plurality of hinged pads moveable between extended and retracted positions. The pads are moved hydraulically using drilling fluid under the control of a valve arrangement. The valve arrangement is designed to permit control over the pads such that, when desired, the pads can be moved to their extended positions in turn as the bias unit rotates. By appropriate control over the pads, the bias unit can be operated to apply a sideways load on the drill bit which in turn will cause the formation of a curve in the well bore being drilled. The orientation of the curve will depend upon how the bias unit is controlled.
It has been found that a number of factors must be taken into account when controlling the operation of a rotary steerable system. For example, the rate of change of direction of the bore hole being formed in response to the application of a given command signal to the bias unit depends upon several factors associated with the drilling system, for example rotary speed, weight on bit, rate of penetration and several factors associated with the formation being drilled, for example the dip and azimuth of bedding planes. As a consequence, it is common for well bores drilled using steerable drilling systems to deviate from their desired paths. Such well bores may be of tortuous form containing many dog legs. Depending upon the orientation of the curves formed in the well bore, water or gas may tend to collect in the curves. Such accumulation of water or gas may impair subsequent use of the well bore in the extraction of oil.
It is an object of the invention to provide a control method for use with a steerable drilling system, the method simplifying control of the drilling system.
According to the present invention there is provided a method of controlling the operation of a steerable drilling system comprising the steps of:
inputting parametric model data representative of drilling conditions;
inputting data representative of a desired drilling direction; and
using the parametric model data and the data representative of the desired drilling direction in controlling the operation of the steerable drilling system.
The parametric model data is conveniently derived using data collected, in real time, during drilling. The parametric model data may include data representative of one or more of the following parameters: weight on bit, rotational speed, rate of penetration, torque, pressure, inclination, dip and azimuth of bedding planes or other formation characteristics, hole curvature/gauge or other geometric conditions, bit type and condition, and errors in instrumentation readings.
The use of such a system is advantageous in that compensation can be made for the operating conditions, thus the risk of supplying the drilling system with instructions to drill a curve of too tight or too small a radius of curvature or of too great or small a length in a given direction can be reduced, thus permitting the drilling of a well bore of less tortuous form.
The parametric model data and data representative of the desired drilling direction may be used directly in controlling the operation of the drilling system. Alternatively, an output signal may be produced which is used to control a display to provide an operator with information for use in controlling the operation of the drilling system. The display may be in a graphic form, for example in the form of a graph of build rate response against turn rate response upon which is plotted an envelope indicating the achievable responses for one or more sets of operating conditions.
With such a display, an operator will be able to see whether it is possible to steer the drill bit of the drilling system in a given direction under one or more sets of operating conditions. The operator may then be able to modify one or more of the operating conditions over which he has some control to ensure that the operating conditions under which the drilling system is operating are such as to permit steering of the drill bit in the desired direction.